Such pipe joints are used, for example, in oil-well tubing to form drill strings composed of a series of pipe sections with threaded male and female extremities at opposite ends. The threaded peripheral surfaces of these extremities are usually of frustoconical shape and terminate in annular lands, namely an end face on the male extremity or plug and an inner shoulder on the female extremity or socket, abutting each other in a final position of assembly. In some instances, unthreaded annular zones in the vicinity of these lands may coact in the fully assembled position to form a tight seal therebetween; usually, the sealing zone on the inner socket surface is substantially frustoconical while the confronting zone on the outer plug surface is either convex or frustoconical.
The joints so formed are subjected to considerable stress, e.g. when used in an oil well in which pipe sections of a length of about six meters are to be assembled into a string several kilometers long. To absorb these stresses, the joint should be formed by direct metal-to-metal contact without the interposition of any more readily deformable sealing member.
According to German printed specification No. 1,533,619, for example, the annular shoulder of the socket has a smooth frustoconical surface converging toward the entrance end of that socket at a large obtuse vertex angle; the annular plug face coacting therewith is of complementary frustoconical configuration. When the two extremities are threadedly interfitted under pressure, the interengagement of these frustoconical surfaces tends to expand the plug face in a radial direction with resulting enhancement of the sealing effect between the confronting peripheral zones. The extent of this expansion, however, is determined by the relative torque applied to the two interfitted pipe sections and depends inter alia on such uncontrollable factors as surface roughness or possible irregularities and the presence of an intervening lubricant film. These considerations also apply to an assembly of the type disclosed in German printed specification No. 1,525,928 in which the shoulder of the socket is subdivided into two concentric annular regions, the outer region being frustoconical with a considerably smaller vertex angle pointing away from the entrance end of the socket. The unthreaded peripheral zone of the inner socket surface immediately adjoining the latter region is separated from the confronting unthreaded zone of the plug by a certain annular clearance into which the material of the plug may penetrate upon the radial expansion of its end face.